In evolutionary terms when did humans first understand time in the sense that we could plan ahead?

Very early on, if not before we were human. Chimps pile their anvil (flat rock ) and hammers (rounded/pointed rock/piece of wood ) around trees that they will be cracking nuts from in advance of the nuts, so they seem to have a rudimentary sense of time and are planing ahead somewhat.

Many advanced mammals seem to have a sense of time in limited to greater degrees, so humans probably had this sense from the time of H. erectus.

Second answer

I agree that it probably predates humans. Regarding chimps, another example is a male individual at the Furuvik Zoo in Sweden who stored a hidden cache of projectiles so that he could later throw them at visitors. You can read more about it in the paper "Spontaneous Innovation for Future Deception in a Male Chimpanzee."

If both humans and chimps have a concept of time, it's very possible that our common ancestor who lived around 7 million years ago had it as well. This would have provided a survival advantage.